UTZ-Notes PDF

HISTORY OF ULTRASOUND INFRASOUND  1918 - Sound Navigation and  Below 16 Hz Ranging was used  Early 1950’s –Water bath ULTRASOUND GENERATORS immersion technique  Late 1950’s – First contact TYPES OF ULTRASOUND compound B-scann...

HISTORY OF ULTRASOUND INFRASOUND  1918 - Sound Navigation and  Below 16 Hz Ranging was used  Early 1950’s –Water bath ULTRASOUND GENERATORS immersion technique  Late 1950’s – First contact TYPES OF ULTRASOUND compound B-scanner WAVES  1970’s – Gray scale imaging  Longitudinal/Compression  Mid 1970’s – Real time Waves scanning systems  Transverse/Shear Waves  1980’s – Doppler technique  Surface/Rayleigh Waves SOUND ACOUSTIC VARIABLES  A mechanical energy  Period (T): the time taken for  Requires a vibrating object to one complete cycle to occur (s produced or µs)  Cannot travel through a  Wavelength (λ): length of vacuum space over which one cycle occurs (m or mm) ULTRASOUND  Amplitude (Depth): the  High frequency sound waves maximum displacement that  Above 20,000 cycles per occurs in an acoustic variable second (20 kHz)  Frequency: cycle per second  Inaudible to humans (Hz)  Used to scan tissues of the body  Velocity: frequency times  Ultrasound Pulse: 2-10 MHz wavelength  Pulse Duration: 1 microsecond  Pulse Repetition: 1000 PIEZOELECTRIC CRYSTALS times/second  Generate ultrasound waves  Capable of changing electrical AUDIBLE SOUND signals into mechanical  16 Hz to 20,000 Hz (ultrasound) waves PIEZOELECTRIC CRYSTAL AS REAL-TIME TRANSMITTER OF SOUND  Shows movement as it occurs  Converting electrical energy into mechanical energy (sound) M-MODE  Shows movement as a function PIEZOELECTRIC CRYSTAL AS of time TRANSMITTER OF SOUND  Used in cardiac scanning  Converting mechanical energy (sound) into electrical energy DOPPLER ULTRASOUND  Demonstrates and measures NOTE: blood flow  Small crystal diameter o Increased beam DOPPLER EFFECT divergence  The change in apparent  Larger crystal diameter frequency of a wave as a result o Decreased beam of relative motion between the divergence observer and the source  Stationary Reflector: reflected DIFFERENT MODES OF echoes are the same as the ULTRASOUND transmitted waves  Reflector that Moves Closer: A-MODE reflected echoes are higher than  Echoes are shows as peaks the transmitted echoes  Distance between various  Reflector that Moves Away: structures can be measured reflected echoes are lower than  Used to build two-dimensional the transmitted echoes B-mode image BASIC TYPES OF DOPPLER B-MODE ULTRASOUND UNIT  Two-dimensional images in which the echo amplitude is 1.) Continuous Wave Doppler Unit depicted as dots of different  Ultrasound is continuous brightness  Measures high velocities accurately   No depth resolution WAVELENGTH  The length of a single cycle of 2.) Pulsed Wave Doppler Unit the ultrasound wave  Ultrasound is transmitted in  Inversely proportional to the pulses frequency Single element  With good depth resolution  Determines the resolution of  Measures the speed of the the scanner blood in a particular vessel  Higher the frequency, the  Cannot measure high blood shorter the wavelength velocities in deep vessels  High velocities may be wrongly FOCUSING displayed as low velocities  Adjustment of the ultrasound beam 3.) Colour Doppler Unit  To improve resolution  Shows different flow-velocities  May be electronic or by a lens in different colours attached to the transducer 4.) Duplex Doppler System AMPLIFICATION  Combination of a B-mode and  Done by the time-gain- Doppler system compensation (TGC) amplier  Allows the Doppler beam to be  To compensate for ultrasound directed accurately at any attenuation in any part of the particular blood vessels body  To improve the quality of the WAVE PROPAGATION final image  The transmission and spread of ultrasound waves to different BOUNDARIES tissues  The line at the periphery of two  Average Propagation for Soft tissues which propagate Tissues: 1540 m/s ultrasound differently  Average Propagation for Soft  The zone of echoes at the Tissues: 4620 m/s interface PIEZOELECTRIC EFFECT  Low density substance – low  Piezein – “press or pressure” acoustic impedance  Ability of a material to generate  Formula: Z=pc an electrical charge un response o p = density of material to applied pressure (kg/m3) o c = speed of sound (m/s) PIEZOELECTRIC MATERIALS o Z = acoustic impedance  Crystalline materials composed (rayls) of dipolar molecules  Quartz – naturally occurring SUBSTANCE Z SPEED crystals (m/s)  Lead zirconate titanate – man Air 0.0004 330 Fat 1.38 1450 made ceramic Water 1.48 1480  Natural Materials: Blood 1.61 1570 o Quartz Kidney 1.62 1560 o Tourmaline Soft Tissue 1.63 1540 o Rochelle Salt Liver 1.65 1550  Synthetic Materials: Muscle 1.70 1580 o Lead zirconate titanate Bone 7.80 3500 PZT (crystal) 30 3870 (PZT) o Barium titanate ACOUSTIC IMPEDANCE AND o Lead metaniobate REFLECTION o Ammonium dihydrogen  Substances with same phosphate acoustic impedance: o Lithium sulphate o 100% energy transmission ACOUSTIC IMPEDANCE o No reflection  Property of a substance  Substances with a small  Describes how the particles of difference in acoustic that substance behave when impedance: subjected to pressure wave o 95% energy transmission  High density substance – high o 5% reflection acoustic impedance   Substances with a large  Best suited to image deep lying difference in acoustic structures impedance:  3.5 MHz o 1% energy transmission o 99% reflection 3.) Convex Transducer  Wide fan-shaped TRANSDUCER/PROBE  Useful for all parts of the body  A device which converts one  Except for specialized form of energy to another echocardiography  Converts electrical energy into ultrasound waves and vice 4.) Phased Array Transducer versa  Flat faced transducer  Contains piezoelectric crystals  Wide field of view o Transmit ultrasound  useful in cardiac and cranial beam ultrasound o Receive reflected echoes COMPONENTS AND TRANSDUCERS/SCANNING CONSTUCTION OF A TYPICAL PROBES TRANSDUCER  The most expensive part of any ultrasound unit 1.) PHYSICAL HOUSING  Contains all individual 1.) Linear Array Transducer components  Parallel scan lines  Provides the necessary  Rectangular field of view structural support  Vascular, small parts and  Acts as an electrical and musculoskeletal applications acoustic insulator  Above 4 MHz 2.) ELECTRICAL 2.) Sector/Curvilinear Array CONNECTIONS Transducer  Formed in front and back of the  Provides wide field of view crystal  Most useful in abdominal and  Made of thin film of gold or obstetric scanning silver 3.) PIEZOELECTRIC  Controls the length of ELEMENTS vibrations from the front face  Crystalline minerals that  Improves axial resolution generate voltages when  Materials: subjected to a mechanical force o Plastic or epoxy resin  Piezein – “to press or squeeze” o Cork  Piezoelectric Effect – o Rubber discovered by Jacques and o Araldite loaded with Pierre Curie tungsten powder  Thinner Piezoelectric Materials o Higher resonant 5.) ACOUSTIC LENS frequencies  Reduce the beam width of the transducer FREQUENCY  Improve image resolution  Affects the quality the  Width of the Beam: ultrasound image determines lateral resolution  Higher Frequency  Lateral Resolution: the ability o Shorter wavelength to resolve structure across or o Better Resolution perpendicular to the beam axis o Lower Penetration  Materials: o Higher Absorption o Aluminum  Lower Frequency o Perspex o Longer wavelength o Polystyrene o Poor Resolution o Higher Penetration 6.) IMPEDANCE MATCHING o Lower Absorption LAYER o Sandwich between the 4.) BACKING/DAMPING piezoelectric crystal and the MATERIALS patient  Shortens the ultrasound pulse o Chosen to improved length transmission into the body  Eliminates the vibrations from the back face BANDWIDTH 3.) Pediatric Ultrasound o The range of frequencies  5.0 MHz transducer: for contained within an ultrasound children pulse  Focused at 5-7 cm o Wide Bandwidth:  Sector transducer of 7 MHz: o Shorter spatial pulse o Neonatal brain scans length o For adult testis and neck o Wider range of  Focused at 4-5 cm frequency o Narrow Bandwidth: ULTRASOUND BEAM o Longer spatial pulse  Area through which the sound length energy emitted from the o Narrower range of ultrasound transducer frequency  Three dimensional and symmetrical around its central CHOOSING THE APPOPRIATE axis TRANSDUCER TWO REGIONS OF 1.) Obstetric Ultrasound ULTRASOUND BEAM  Linear or convex transducer 1.) Near Field/Fresnel zone  3.5 MHz: better in later 2.) Far Field/Fraunhofer zone pregnancy  Increasing Frequency  5.0 MHz: best during early o Longer near field pregnancy o Less far field divergence  Focused at 7-9 cm  Narrow Crystal Diameter o Narrower near field o More far field divergence 2.) General Purpose Ultrasound  Thin Crystal  Sector or convex transducer o Decreased near field  3.5 MHz o Increased far field  Focused at 7-9 cm  Thick Crystal o Increased near field o Decreased far field BEAM INTENSITY SPATIAL RESOLUTION  The power (measured in watts)  Detail Resolution flowing through a unit area  The ability to display two structures situated close SIDE LOBES/GRATING LOBES together as separate images  Lobes at various angles to the  Higher Frequency: main beam o Better resolution  Approximately 15% of the o Lower penetrability energy in the beam o Higher absorption  Cause a degradation of lateral  Lower Frequency: resolution o Poor resolution o Higher penetrability BEAM WIDTH o Lower absorption  The dimension of the beam in the scan plane TWO COMPONENTS OF  Affects the spatial resolution SPATIAL RESOLUTION  Narrow Beam Width o Better spatial resolution 1.) AXIAL RESOLUTION  Longitudinal, Linear, Depth or SLICE THICKNESS Range  Three dimensional volume  The ability to distinguish two displayed as a two dimensional objects parallel to the image ultrasound beam  Depends upon the spatial pulse RESOLUTION length and wavelength  The ability of an imaging  Short Spatial Pulse Length: system to differentiate between good axial resolution structures  Longer Spatial Pulse Length:  Spatial Resolution: resolution poor axial resolution in space  Contrast Resolution: 2.) LATERAL RESOLUTION resolution of gray shades  Azimuthal, Transverse,  Temporal Resolution: Angular or Horizontal resolution in time   The ability to distinguish two waves as they pass through objects perpendicular to the tissues ultrasound beam  Unit: decibels per centimeter  Depends upon the beam diameter FIVE MAIN PROCESSES THAT  Smaller Beam Width: better CAUSE ATTENUATIONS lateral resolution  Larger Beam Width: poor 1.) ABSORPTION lateral resolution  Occurs when ultrasound energy is lost to tissues by its CONTRAST RESOLUTION conversion to heat  The ability of the imaging  Main factor causing attenuation system to differentiate between  Higher Frequency: body tissue and display them as o Greater amount of different shades of gray absorption  Optimized by using the correct  Bone: higher absorption overall gain coefficient  Increasing protein content gives TEMPORAL RESOLUTION increasing absorption  Frame Rate o Blood –> Fat –> Nerve –  The ability of the imaging > Muscle –> Skin –> system to display events which Tendon occurs at different times as –> Cartilage –> Bone separated images  Best Absorption: tendon,  Higher Frame Rate: better ligament, fascia, joint capsule temporal resolution & scar tissue ULTRASOUND INTERACTIONS 2.) REFLECTION AND ATTENUATIONS  Occurs when two large structure of significantly ATTENUATION different acoustic impedance  Decrease in the intensity and form an interface amplitude of the ultrasound   Occurs when a sound wave 1.) REVERBERATION strikes an object that is larger  Comet tail than the wavelength  The production of spurious or false echoes due to repeated 3.) SCATTERING reflections between two  Occurs when an ultrasound interfaces with a high acoustic wave strikes a boundary or impedance mismatch interface between two small  The presence of two or more structures strong reflecting surfaces  Occurs when a sound wave  Often occur at: strikes an object that is equal to o Skin-transducer interface or smaller than the wavelength o Gas surface and transducer 4.) REFRACTION  Prevention/Elimination:  Occurs when the beam o Increase the amount of encounters an interface between gel used two different tissues at an o Used a stand-off gel pad oblique angle o Reduce the gain  The beam will be deviated as it o Move the position of the travels through the tissue transducer  Occurs due to difference in wave velocity across an 2.) ACOUSTIC SHADOWING interface between two materials  Caused by highly attenuating structure 5.) DIVERGENCE  Often occur at:  Occurs when the beam travels o Soft tissue and gas through tissue and it will o Soft tissue and bone or diverge due to diffraction calculus effects o Calcified mass ULTRASOUND ARTIFACTS 3.) ACOUSTIC ENHANCEMENT  A structure in an image which  Caused by weakly attenuating does not directly correlate with structures actual tissue being scanned   Often occur at: have arisen form the central o Distal to fluid-filled axis of the main lobe urinary bladder,  Appearance can give rise to a gallbladder or cyst false diagnosis o Fluid-filled mass  Inherent characteristic of the transducer 4.) EDGE SHADOWING  Combination of refraction and 8.) MIRROR IMAGE ARTIFACT reflection occurring at the  Caused by specular reflection edges of rounded structures of the beam at a large smooth interface 5.) BEAM WIDTH ARTIFACT  Often seen in:  Variations of all echoes o Fluid-air interface returning to the transducer o Diaphragm  Prevention/Elimination: o Correct positioning of 9.) DOUBLE IMAGE ARTIFACT the focal zone  Caused by refraction of the beam 6.) SLICE THICKNESS  Often occur at: ARTIFACT o Rectus abdominis muscle  Occurs due to the thickness of  Prevention/Elimination: the beam o Move the transducer  Dependent upon beam slightly to one side to angulation avoid the junction of  Often seen in: rectus abdominis muscle o Transverse view of the urinary bladder 10.) EQUIPMENT-GENERATED  Inherent characteristic of the ARTIFACT transducer  Caused by incorrect use of the equipment control 7.) SIDE LOBE ARTIFACT  Echoes generated by side lobes “Passing the Board Exam is your assumed by the transducer to main purpose to learn” 05/13/14